1. Field of the Invention
The invention relates to an optical module having lenses and a optical modulation device mounted on a surface of a silicon substrate, and a method of manufacturing the optical module.
2. Description of the Related Art
Many optical modules having lenses and a optical modulation device mounted on a surface of a silicon substrate are proposed. Some of them are disclosed in the following Japanese Patent Publication References.
JP H08-201660A (hereinafter referred as the Reference 1)
JP H09-211274A (hereinafter referred as the Reference 2)
JP 2004-093660A (hereinafter referred as the Reference 3)
According to the Reference 1, an optical module having a following structure is disclosed. The optical module disclosed in the Reference 1 includes a first V-letter shaped groove and a second V-letter shaped groove perpendicular to the first groove, both of which are formed in a silicon substrate. One end of an optical fiber is fixed at the first groove, and a light emitting device is mounted along the optical axis of the optical fiber with the relatively soft accuracy. A ball lens is placed in the second groove. In order to adjust the optical relationship among the light emitting device, the ball lens and the optical fiber, the ball lens is moved along the second groove while the light emitting device is in operation. The light beam emitted from the light emitting device is converged by the ball lens, and is entered into the optical fiber. As a result, by measuring the coupling condition among the light emitting device, the ball lens and the optical fiber, the optical relationship among them is adjusted in the most appropriate condition.
In the field of the long distance optical communication, it is required to use a transmitter, which has small wavelength chirp, as a lighting source, because the wavelength dispersion characteristics, which are inherent in the optical fiber as the transmission medium, deteriorates the communication quality in which a little fluctuation of the communication wavelength makes large distortion to the transmittal waveform. Even if a Distributed Feed-Back type Laser device (hereinafter called a DFB Laser device) is used as a lighting source, an external modulator system such as an Electro-Absorption (EA) semiconductor modulator or a Mach-Zehnder type modulator using Lithium Niobe Oxide (LiNbO3) or Indium Phosphide (InP) compound semiconductor is generally used because an oscillation wavelength of the DFB Laser device is easily fluctuated in a direct modulation system in which the injected current is directly transformed.
Under the external modulator system, because the modulation control of the input/output of light is performed by the external devices, the injected current of the DFB Laser device can be used as the constant optical output without transforming injected current. Thus, a single waveform oscillation having small wavelength chirp can be fulfilled. In recent years, a device in which a DFB Laser device and an EA modulator element are monolithically-integrated is frequently used so that an assembling cost or an area to be assembled can be reduced.
On the other hand, in a dense wavelength division multiplexing (DWDM) transmitting system, which is superior for the expandability of the communication capacity, it is necessary to suppress the cross-talk between the light signals, each of which is adjacent to each other with an extremely narrow wavelength interval, due to the trend of the multiple wavelength by segmentalizing the band wavelength. For this reason, the stabilization of each signal wavelength becomes one of the most important issues to be resolved. However, when the number of the communication wavelength is increased because of the segmentation of the optical wavelength, it is required to prepare the devices of the particular number, which is the same as the number of the wavelength sued in the system, on the basis that a band-gap wavelength of an absorber layer in the EA modulator element is designed to match up with each communication wavelength and is provided in the modulator in which the EA modulator element is used. As a result, increasing the cost for the development, the manufacture and the management of the stocks is unavoidable. Thus, it is considered that there is a limit to apply the EA modulator element in the DWDM transmitting system. Furthermore, while the oscillation wavelength of the DFB Laser device is changed in the rate of 1 Å/° C., the bang-gap wavelength of the EA modulator element is changed in the rate of 4˜5 Å/° C. Thus, in order to avoid shifting a wavelength-detuning, which means that the band-gap wavelength of the absorber layer in the EA modulator element matches up with the oscillation wavelength of the DFB Laser device, it is required to control the temperature of each of the DFB Laser device and the EA modulator element, rigorously. To resolve this problem, a modulator using a Mach-Zehnder type modulator element starts to garner attention.
Since the Mach-Zehnder type modulator element has small wavelength chirp, and it can covers a wide range of wavelength domain as a single device, it is suitable for the long distance and multiple-wavelength transmission system. By the monolithic or the hybrid integration of the DFB Laser device with the Mach-Zehnder type modulator element, it is possible to reduce the number of the device group being prepared for the number of the wavelength used in the system. As a result, it is possible to reduce the cost for the development, the manufacture and the management of the stocks.
The hybrid integration structure of the DFB Laser device with the Mach-Zehnder type modulator element serve as a typical example used in the long distance and multiple-wavelength transmission system is explained as follows with reference to FIG. 6. FIG. 6 is a side view showing a frame format of the conventional optical module disclosed in the Reference 2.
The optical module shown in FIG. 6 is a device hybridly integrating a DFB Laser device 1 with a Mach-Zehnder type modulator element 2. The DFB Laser device 1 mounted on a first carrier 3 and the Mach-Zehnder type modulator element 2 mounted on a second carrier 4 are optically coupled to each other by a first lens 5 having a holder and a second lens 6 having a holder. The first lens 5 and the second lens 6 are centered along their cores in the three axis directions on a first lens core adjuster 7 and a second lens core adjuster 8, respectively, and then, these adjusters 7 and 8 are fixed by the YAG Laser weld on a stainless base carrier 9.
Further, a tube-shaped isolator 10, which passes light in one direction, is disposed between the first and the second lenses, and is fixed on supporting member 11 by the YAG Laser weld. Moreover, a compensation lens 12 having a holder, which optically adjusts the slight dislocation of the centering for the cores of the lenses 7 and 8, is disposed between the lenses 7 and 8. The compensation lens 12 is fixed by the YAG Laser weld on the stainless base carrier 9 via a compensation lens adjuster 13 after the adjustment of the dislocation of the centering for the cores of the lenses 7 and 8. As described in the Reference 2, when the compensation lens 12 having a large curvature radius is used, it is possible to perform the fine adjustment of the angle of the focused laser beam with a rough centering process. Thus, by suing the compensation lens 12 having a large curvature radius, the adjustment of the slight dislocation can be easily fixed.
After a third lens 14 having a holder is centered along its cores with the cores of the first and the second lenses 7 and 8 in the three axis directions in order to couple an optical signal outputted from the Mach-Zehnder type modulator element 2 with an optical fiber 16 having a built-in lens, which is disposed outside a package 15, it is fixed by the YAG Laser weld on the stainless base carrier 9 via a third lens adjuster 17. The stainless base carrier 9 is fixed by solder on a thermal electro cooler 18 disposed in the package 15
The DFB Laser device 1 outputting a uniform light is thermally controlled by thermal electro cooler 18 in order to oscillate a single waveform constantly. The Mach-Zehnder type modulator element 2 is electrically controlled by a high-speed modulation signal outputted from a driver IC, and is functioned as a shutter either for passing through or blocking the laser light from the DFB Laser device 1.
However, the optical module shown in FIG. 6 has following issues. First, since a spot size of the light in each of the DFB Laser device 1 and the Mach-Zehnder type modulator element 2 is very small such as around 1 μm and since a tolerance of the dislocating of the centering for the cores of them is very strict, the accuracy required for centering the cores of lenses should be within an accuracy of sub-micrometers. In fact, because of the use of the compensation lens 12, the accuracy is eased to an accuracy of micrometers. However, since the range of the adjustment by the compensation lens 12 is not so wide, the DFB Laser device 1, the first lens 5, the second lens, and the Mach-Zehnder type modulator element 2 should be disposed to an accuracy of micrometers in order to obtain the fine optical coupling efficiency.
It is not easy for the hybrid structure having a plurality of components illustrated in FIG. 6 to dispose the components at the locations to an accuracy of micrometers. For this reason, new assemble process utilizing an image or a marker-recognition is introduced, or the strict management as to the size of each component to be mounted is required. As a result, the cost for the accurate process or the inspection of the components or the cost for accurate mounting the components increases.
The second issue relates to the method of fixing the lenses. As described above, the first lens 5 and the second lens 6 are fixed by the YAG Laser weld on a stainless base carrier 9 via each of the adjusters 7 and 8 after they are centered along their cores in the three axis directions. Because of the inherency of the YAG Laser weld, the melted metal is condensed at the time of the natural cooling so that it is not easy to place the lenses at the location to an accuracy of micrometers. Thus, the dislocation of the centering for the cores of the lenses 7 and 8 caused at the time of the YAG Laser weld is adjusted by the compensation lens 12. However, since the compensation lens 12 having the large curvature radius is lens-shaped, while it has ability for fine adjustment of the angle of the focused laser beam with a rough centering process, it has little ability for focusing the light. Thus, the compensation lens 12 cannot adjust the dislocation in the optical axis direction. For this reason, it is still required to dispose the lenses in the optical axis direction to an accuracy of sub-micrometers.
In order to avid these issues, an optical module disposed in the Reference 3 is proposed. According to the optical module disclosed in the Reference 3, a V-letter shaped groove is formed at a surface of a silicon substrate, and a spot shape changeable element, a first and a second lens elements, an EA modulator, a Laser diode, and optical fiber are mounted along the V-letter shaped groove. The Laser diode and the EA modulator are optically coupled by the first lens element, and the EA modulator and the optical fiber are optically coupled by the spot shape changeable element and the second lens element. The EA modulator includes a waveguide structure and it has a rectangularily-shaped core at its cross-sectional view. The spot of the light outputted from the core is elliptically-shaped.
The divergent light having the elliptically-shaped spot outputted from the Laser diode is focused by the first lens element and the focused divergent light still having the elliptically-shaped spot is injected into the core of the EA modulator. The divergent light having the elliptically-shaped spot outputted from the core is changed to the parallel light having the circularly-shape spot, and then the parallel light having the circularly-shape spot is transformed into the convergent light by the second lens element. Finally the convergent light is injected into the optical fiber.
According to the optical module disclosed in the Reference 3, since the spot shape changeable element and the lens elements are disposed between the EA modulator and the optical fiber, the parallel light having the circularly-shape spot, which is changed from the divergent light having the elliptically-shaped spot outputted from the core of the EA modulator, can be injected into the optical fiber. As a result, the coupling efficiency can be dramatically improved. Further, since the spot shape changeable element includes an adjustment member whose shape is suitable for the silicon substrate, no holder is required for the lenses and the lenses are easily mounted in the V-letter shaped groove in a short amount of time. Thus, the cost can be reduced. Moreover, the lenses disclosed in the References 1 and 2 are used, it is required to have a wiring distance sufficient to apply the bias voltage to the EA modulator. However, according the optical module disclosed in the Reference 3, the wiring distance can be the same as that in the case that the spot shape changeable element and the lens elements are not used. Thus, the deterioration of the modulation characteristic can be avoided. As a result, the optical components can be effectively coupled to each other.
However, the optical module disclosed in the Reference 3 has another issue as follows. Generally, after mounting the EA modulator and the Laser diode on the silicon substrate, burn-in tests are performed to detect an incipient failure on the EA modulator or the Laser diode in advance. Since the EA modulator and the Laser diode are mounted on the single silicon substrate, the device that both of the first burn-in test for the relationship between the Laser diode and its wire and the second burn-in test for the relationship between the EA modulator and its wire are passed can only be judged as the non-defective products so that the yield rate is deteriorated.